It is well-known how to transmit either voice signals or data signals over a single transmission channel. In U.S. Pat. No. 3,549,814 issued Dec. 22, 1970 to R. J. Jaeger, Jr. et al, there is disclosed a pulse code modulation system in which each channel can simultaneously transmit a voice frequency message, and at the same time, all channels together cooperatively transmit a data message. Specifically, there is disclosed a system in which a 24-bit word is transmitted simultaneously with twenty-four different voice signal words, by assigning the eighth bit-position of each eight-bit voice signal word for one of the aforesaid data bits.
Where telecommunications users would like to transmit data signals, such as graphics, and simultaneously engage in a telephone conversation, two telephone lines would be necessary. Such a method would be expensive. Alternatively, the data signals could be transmitted first over a telephone line, and the telephone conversation could follow, or vice versa. This approach would be an inconvenient method for some applications. It is therefore desirable to provide method and means whereby users can engage in a telephone conversation and, simultaneously, transmit data signals using a single telephone line, thereby making such systems both economically attractive and convenient to use.